Highlights from Recent Cancer Literature
Journal: Cancer Research (2012)

Rhim AD, Mirek ET, Aiello NM, Maitra A, Bailey JM, McAllister F, et al. EMT and dissemination precede pancreatic tumor formation. Cell 2012;148:349–61.The possibility that malignant cells may disseminate even before the primary lesion is detected has great biologic and clinical significance. In a recently published article, Rhim and colleagues make an elegant and important contribution to our understanding of early events in metastases. A widely accepted view of metastases, where primary tumors acquire mutations that promote invasion and dissemination at a late stage of development, is challenged by the appearance of metastases many years after resection of small primary cancers or by metastases from unknown primary tumors. In addition, there is evidence for malignant cells in the bone marrow of patients who have early in situ cancers elsewhere. Using a genetic mouse model of pancreatic cancer, Rhim and colleagues developed a sensitive cell lineage–labeling system that allowed detection of pancreatic epithelial cells in the blood and characterization of their invasive and tumor-initiating properties. They found that the “tagged” pancreatic epithelial cells invaded and entered the circulation before malignant lesions could be found in the pancreas. The labeled cells had an epithelial–mesenchymal (EMT) phenotype, which was maintained in the blood stream. The cells also exhibited stem cell–like properties and seeded the liver, a common site of pancreatic cancer metastases. It was interesting that the disseminating cells were frequently found at inflammatory foci, pancreatitis increased the number of invasive pancreatic cells, and the immunosuppressive drug dexamethasone stopped their dissemination. To the many known associations between inflammation and cancer, we can therefore add the ability to increase EMT in vivo and early dissemination of malignant cells with invasive potential and tumor-initiating properties. This work has particular relevance to patients at high risk of pancreatic cancer—if cells with malignant potential circulate before cancer is detected, there may be a window of opportunity for prophylactic treatment with anti-inflammatory drugs. Epidemiologic studies have shown that long-term use of anti-inflammatory drugs may reduce mortality from some cancers. Rhim and colleagues may have provided one explanation for this.Gao D, Joshi N, Choi H, Ryu S, Hahn M, Catena R, et al. Myeloid progenitor cells in the premetastatic lung promote metastases by inducing mesenchymal to epithelial transition. Cancer Res; Published OnlineFirst January 26, 2012; doi:10.1158/0008-5472.CAN-11-2905.The cellular and molecular mechanisms that underlie the establishment of the metastatic niche in distant target organs are still poorly understood. In this article, Gao and colleagues report new insights that shed light on the identification of important determinants for the progression of micrometastases to macrometastases. They used a spontaneous breast cancer mouse model (MMTV-PyMT transgenic mice) to show that specific lineages of bone marrow cells, mainly CD11b+Gr1+ myeloid progenitor cells, are recruited to premetastatic lung niches. Using an unbiased gene expression profiling approach, they discovered that the extracellular matrix proteoglycan versican was highly expressed in a specific subset of these myeloid progenitor cells (CD11b+Ly6Chigh). They also show by a loss of function approach that versican activity enhances cell proliferation and the progression of metastatic growth. Furthermore, Gao and colleagues use metastatic human breast cancer cells, MDA-MB-231, to show in vitro and in vivo that versican stimulates mesenchymal–epithelial transition (MET), possibly through blockade of the TGF-β–Smad2 pathway. Others in the field have proposed that METs are necessary to recapitulate features of the primary tumor at secondary sites. Gao and colleagues establish that induction of MET enhances metastatic growth. Importantly, they have translated their findings directly into metastatic lung and breast cancer patients, showing that levels of versican were increased in these human patient samples. Furthermore, flow cytometry analysis shows that versican is expressed specifically in CD11b+CD33+ cells, which comprise the monocytic population in these human specimens. This study opens a new avenue of research into the development of therapeutics for metastatic disease that could selectively target tumor-associated myeloid cells or directly inhibit versican.Zhao B, Li L, Wang L, Wang CY, Yu J, Guan KL. Cell detachment activates the Hippo pathway via cytoskeleton reorganization to induce anoikis. Genes Dev 2012;26:54–68.The gene encoding Yes-associated protein (YAP) is a human oncogene. YAP is inhibited by the Hippo tumor-suppressor pathway via phosphorylation. Zhao and colleagues report on their investigation of relationships of the Hippo pathway YAP phosphorylation/localization and anoikis. They first investigated YAP phosphorylation, localization, and activity as a function of cell attachment to the extracellular matrix. Upon cell attachment, YAP was rapidly dephosphorylated, with reciprocal phosphorylation occurring upon detachment. In addition, YAP shuttled into the nucleus within 80 minutes of cell attachment. The authors investigated functional implications by showing induction of YAP target genes, CTGF and Cyr61, as a result of cell attachment. They then showed that focal adhesion was not required for cell attachment–induced activation of YAP. Comparing cells attached to integrin ligands (which induces focal adhesion) versus polylysine (which does not induce focal adhesion), they observed similar levels of YAP activation and nuclear translocation, indicating that this process was independent of focal adhesion formation. Using inhibitors of actin cytoskeleton and microtubule reorganization, they found that prevention of actin polymerization blocked attachment-induced YAP dephosphorylation. In contrast, whereas blocking microtubule polymerization had no affect on attachment-induced dephosphorylation of YAP, blockage of microtubule polymerization inhibited detachment-induced dephosphorylation. Cytoplasmic-nuclear localization was concordantly affected as a result of actin and microtubule reorganization. The authors then examined the Hippo pathway in more detail. Using the YAP 5SA allele, which lacks Lats serine phosphorylation sites and is unresponsive to the Hippo pathway, they showed that this allele localized to the nucleus in a manner independent of microtubule integrity, suggesting that Lats-mediated phosphorylation was required for subcellular localization. Further, knockdown of Lats reduced YAP phosphorylation in a manner independent of cell attachment, indicating that Lats 1 and 2 were the kinases responsible for this effect. Consistent with this model, disruption of microtubule reorganization decreased Lats activity. However, Mst 1 and 2, which are upstream from Lats in the Hippo pathway, were not affected, indicating that cell attachment regulated the Hippo pathway downstream from Mst 1 and 2 and upstream of Lats 1 and 2. Finally, because cell attachment regulates anoikis (defined as detachment-regulated apoptosis), the authors investigated the role of the Hippo pathway in this phenomenon. They found that both wild-type YAP and the constitutively active YAP 5SA allele were potent blockers of anoikis. Because the process of cancer metastasis likely involves evasion of anoikis, the authors examined the Hippo pathway in cancer metastasis. They found that YAP knockdown promoted anoikis. In addition, a comparison of localized versus metastatic human prostate cancer tissues revealed a relative downregulation of Lats 1 and 2 in metastatic tissues, suggesting a possible role of the Hippo pathway in metastasis. Overall, these findings provide a novel connection between cell attachment to the extracellular matrix and anoikis through the Hippo pathway and may have implications in therapeutics for metastatic cancer.Barrett LE, Granot Z, Coker C, Iavarone A, Hambardzumyan D, Holland EC, et al. Self-renewal does not predict tumor growth potential in mouse models of high-grade glioma. Cancer Cell 2012;21:1–24.Much has been written about glioma “stem” cells, but controversy remains as to what constitutes and how to mark glioma stem cells. Glioma stem cells are enriched in neurosphere cultures in serum-free media, and the ability of single cells to reform neurospheres is a common assay used to determine self-renewal. In turn, self-renewal in vitro has been linked to tumorigenesis in vivo. Barrett and colleagues examine a central question of the stem cell hypothesis: Is the capacity for self-renewal directly tied to tumorigenesis? Using virally transduced models for glioma, they first show that platelet-derived growth factor–driven murine gliomas exhibit high-level expression of the basic helix–loop–helix transcriptional regulator Id1. Using mice engineered to express a fluorescently labeled Id1 protein, they identify Id1-high and -low populations of tumor cells. Id1-high cells, comprising less than 1% of the total population, also displayed high levels of the stem cell marker Prominin-1 (CD133), whereas ID1-low cells overexpressed progenitor-associated markers, including Olig2. Similar results were found in mutant KRAS-driven tumors. The authors show that self-renewal capacity, as defined by neurosphere formation, was higher in the Id1-high fraction, as compared with the Id1-low fraction. Surprisingly, Id1-low cells were more efficient at transplanting tumors, exhibiting both higher penetrance and shorter latency compared with Id1-high cells. Further, tumor cells from Id1-knockout mice displayed a marked reduction in neurosphere formation but only modest effects on tumorigenesis. In contrast, Olig2 knockdown increased tumor-free survival in mice. Examination of the relationship of Id1 expression with patient outcomes in data from The Cancer Genome Atlas glioblastoma samples showed that Id1-low tumors were associated with significantly worse outcome than Id1-high tumors in the proneural subclass. These findings have important implications in the field of glioma-initiating cells, as cells with progenitor-like properties appear to be more tumorigenic than cells with higher self-renewal capacity. Thus, self-renewal, a central tenet of “stemness,” is not necessarily linked to the ability to form tumors in vivo. While further work is required to explore the generality of these findings in additional model systems and human tumors, this article raises serious questions about stem cell markers and the utility of the widely used self-renewal assay as a measure of gliomagenesis.Tamada M, Nagano O, Tateyama S, Ohmura M, Yae T, Ishimoto T, et al. Modulation of glucose metabolism by CD44 contributes to antioxidant status and drug resistance in cancer cells. Cancer Res; Published OnlineFirst January 31, 2012 doi:10.1158/0008-5472.CAN-11-3024.It is well recognized that the Warburg Effect, or aerobic glycolysis, plays a pivotal role in cancer cell metabolism and survival in concert with deregulated p53, as p53 tightly regulates mitochondrial respiration. However, the distinct mechanism by which tumor cells favorably select glycolysis for fulfilling the energy requirement remains largely unknown. In this study, Tamada and colleagues have shown that CD44, a major adhesion molecule implicated in many significant cellular processes including cell migration, cell growth, and tumor development and metastasis, regulates energy production and cell proliferation in p53-deficient cancer cells. This effect is promoted by inhibition of the activity of pyruvate kinase M2 (PKM2), a major regulator of aerobic glycolysis that targets PKM2. Knocking down CD44 reduced glucose uptake and the pentose phosphate pathway flux accompanied by reduced glutathione activity and increased reactive oxygen species (ROS) production. Moreover, CD44 abrogation sensitized glycolytic cancer cells to antitumor drugs. These data suggest that CD44 may be a regulator of metabolic shift through regulation of PKM2 and ROS protection in tumor cells. Thus, it appears that CD44, widely expressed in various cancer cells and a cell surface marker for cancer stem cells, could be a potential target for therapeutic interventions by modulating energy production in cancer cells.Note: Breaking Advances are written by Cancer Research Editors. Readers are encouraged to consult the articles referred to in each item for full details on the findings described.